Where to obtain flat spring steel?

[Sidney Smith]

Update. I worked on the part this evening and got it at least partially fitted to the lock. Surprisingly, the part has enough springboard to it the way it sits to return the sear bar and serpentine back to the up position. Wondering now whether it even requires hardening. It seems to work. Thoughts?

 

[PhDBrewer]

If you harden & temper it, it will be more responsive and not lose its shape over time. Annealed steel is soft.
Sounds like you got this figured out!
William